In a reducing agent supply device which is arranged in an exhaust passage of an internal combustion engine, there has been disclosed a technique in which the time at which a reducing agent supply valve gets clogged is estimated, and a small amount of reducing agent is injected from the reducing agent supply valve before the estimated time of occurrence of clogging, or an amount of reducing agent increased more than an ordinary amount is injected from the reducing agent supply valve after the estimated time of occurrence of clogging (for example, refer to a first patent document). In the first patent document, the clogging of a nozzle (or injection hole) of the reducing agent supply valve, which serves to inject the reducing agent, is suppressed in this manner.